This invention relates to digital data receivers, of the kind for demodulating a fixed code spread spectrum differential phase shift keyed received data signal, representing a succession of data symbols having a predetermined symbol rate.
Various types of modulation are known for transmitting digital data. One known type of modulation is quadrature phase shift modulation (QPSM), wherein a continuous wave (CW) signal is switched in phase between 0.degree., 90.degree., 180.degree. and -90.degree.. This gives four possible states per symbol, that is, two bits of information are transmitted per symbol. To demodulate such a signal, a coherent reference signal (an oscillator) is required as a phase reference. In order to obviate this problem, it is known to use a differential phase shift keyed signal structure (DPSK). With this signal structure, it is not the absolute phase of a symbol which represents the information, but the phase change between two adjacent symbols, which may be 0.degree., 90.degree., 180.degree. or -90.degree.. To demodulate such a DPSK signal, the phase of two adjacent symbols has to be compared.
It is also known, for data transmission, to employ spread spectrum systems. Spread spectrum communications systems employ a bandwidth which is greater than the data bandwidth. This provides several advantages, including a high resistance to interference, a greater tolerance to multipath signals, and lower spectral power density. One type of spread spectrum system, known as direct sequence spread spectrum (DSSS), effects a "spreading" function by modulating the information by a fixed pseudorandom code. For example, binary 0 and 1 level signals can be modulated, using an EXCLUSIVE-OR type gating device, with the individual bits of the fixed pseudorandom code, to produce a plurality of bits, known as "chips", representing the original binary 0 or 1 level signal. Thus, in the time of one symbol, many chips are present, whereby the spectral occupancy of the transmitted signal is increased, in accordance with the number of chips in one symbol period. In this manner, a so-called processing gain of the DSSS system is achieved.
A data transmission system utilizing spread spectrum encoding and differential phase shift keying modulation is known from U.S. Pat. No. 4,672,658, in an application to a wireless PBX network, used for voice and data communications. The receiver utilized in this known system employs a surface acoustic wave (SAW) matched filter correlator to effect a "despreading" operation, after which a demodulator demodulates the differential phase shift keyed signal. This known arrangement has the disadvantage of a complex and expensive implementation.